Since the discovery of carbon nanotubes in 1991 [Iijima, “Helical microtubules of graphitic carbon,” Nature, 354, pp. 56-58 (1991)] and single-wall carbon nanotubes in 1993 [Iijima et al., “Single-shell carbon nanotubes of 1-nm diameter,” Nature, 363, pp. 603-605 (1993); Bethune et al., “Cobalt-catalysed growth of carbon nanotubes,” Nature, 363, pp. 605-607 (1993)], a substantial amount of research has been carried out involving the synthesis, chemistry, and manipulation of these novel materials. See Ebbesen, “Carbon Nanotubes,” Annu. Rev. Mater. Sci., 24, pp. 235-264 (1994); Zhou et al., “Materials Science of Carbon Nanotubes: Fabrication, Integration, and Properties of Macroscopic Structures of Carbon Nanotubes,” Acc. Chem. Res., 35(12), pp. 1045-1053 (2002); Dai, “Carbon Nanotubes: Synthesis, Integration, and Properties,” Acc. Chem. Res., 35(12), pp. 1035-1044 (2002). The goal of much of this research is to facilitate the exploitation of carbon nanotubes' intriguing properties. See Yakobson et al., “Fullerene Nanotubes: C1,000,000 and Beyond,” American Scientist, 85, pp. 324-337 (1997); Ajayan, “Nanotubes from Carbon,” Chem. Rev., 99, pp. 1787-1799 (1999); Baughman et al., “Carbon Nanotubes—the Route Toward Applications,” Science, 297, pp. 787-792 (2002).
The electronic properties of carbon nanotubes have been shown to be perturbed by bending-induced strain, wherein such perturbations manifest themselves in the form of enhanced reactivity. See Ausman et al., “Nanostressing and Mechanochemistry,” Nanotechnology, 10, pp. 258-262 (1999); and Ruoffet al., “Mechanical Properties of Carbon Nanotubes: Theoretical Predictions and Experimental Measurements,” C. R. Physique, 4 pp. 993-1008 (2003). The electrical properties of carbon nanotubes have also been demonstrated to vary non-linearly when subjected to high pressures, i.e., up to 90 kbar. See Bozhko et al., “Resistance vs. Pressure of Single-Wall Carbon Nanotubes,” Appl. Phys. A, 67, pp. 75-77 (1998).
To date, several experiments [Bezryadin et al., “Multiprobe Transport Experiments on Individual Single-Wall Carbon Nanotubes,” Physical Review Letters, 80, 4036-4039 (1998); Narderi et al., “TITLE?,” Physical Review B, 60, 16334-? (1999); Peng et al., “Chemical control of nanotube electronics,” Nanotechnology, 11, 57-60 (2000); Tombler et al., “Reversible electromechanical characteristics of carbon nanotubes under local-probe manipulation,” Nature, 405, 769-772 (2000)] have studied the effect of mechanical strains on the electronic properties of SWNTs at the nanoscale. Peng et al. have reported that carbon nanotubes have mechanical deformations such as bending, twisting or flattening, and that these influence their electronic properties. Tombler et al. have concluded that the voltage across a single-wall carbon nanotube can be reduced by two orders of magnitude when it is deformed by an AFM tip. Baughman et al. used carbon nanotube films, also called buckypapers, as actuators. See Baughman et al., “Carbon Nanotube Actuators,” Science, 284, 1340-1344 (1999). Results showed that large actuator strains can be achieved by smaller operating voltages compared with ferroelectric and electrostrictive materials.
Electronic perturbations induced by chemical adsorbates have been exploited to produce sensors which respond to the adsorption of a small molecular species with a corresponding change in conductivity and resistance. See Kong et al., “Nanotube Molecular Wires as Chemical Sensors,” Science, 287, pp. 622-625 (2000); Collins et al., “Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes,” Science, 287, pp. 1801-1804 (2000); and Cole et al., U.S. patent application, Ser. No. 10/100,440, filed Mar. 18, 2002. More recently, carbon nanotubes have been used as flow sensors, wherein the fluctuating Coulombic field of a liquid flowing past the nanotubes forcibly drags the free charge carriers of the nanotubes, thereby inducing a voltage in the nanotube sample along the direction of the flow. See Ghosh et al., “Carbon Nanotube Flow Sensors,” Science, 299, pp. 1042-1044 (2003).
SWNTs are Raman active and many researchers have studied the effect of stress or strain on the Raman active modes. Recently, researchers have presented results indicating a Raman shift at ˜1590 cm−1, [Hadjiev et al., “Raman scattering test of single-wall carbon nanotube composites,” Applied. Physics Letters, 78, 3193-3195 (2001); Li et al., “Carbon Nanotube Film Sensor,” Advanced Materials, Submitted (2003)] termed as the G band shift, due to tensile strain in the nanotubes. Similar Raman studies on multi-wall carbon nanotubes have also been reported. See Wagner et al., “Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix,” Applied Physics Letters, 72, 188-190 (1998); and Schadler et al., “Load transfer in carbon nanotube epoxy composites,” Applied Physics Letters, 73, 3842-3844 (1998). Wagner and co-workers have shown that Raman spectroscopy can be used as a probe of stress in polymer composites comprising carbon nanotubes by observing a shift in wavenumber of the disorder-induced Raman D* band (˜2610 cm−1) of SWNTs, which reflects a breathing vibrational mode in graphite, and which has been observed to shift linearly with elastic matrix strain. See Zhao et al., “The Use of Carbon Nanotubes to Sense Matrix Stresses Around a Single Glass Fiber,” Composites Sci. & Tech., 61, pp. 2139-2143 (2001); and Zhao et al., “Direction-Sensitive Strain Mapping with Carbon Nanotube Sensors,” Composites Sci. & Tech., 62, pp. 147-150 (2002). However, Raman spectroscopy is an arduous process requiring sensitive, sophisticated equipment. Moreover, this technique relies on perturbations in the vibrational modes of carbon nanotubes, not merely on perturbations of their electronic structure.
A sensor, capable of detecting and monitoring mechanical stress/strain at the macroscale, which is sensitive to electronic perturbations manifested in electrical, optoelectronic, or luminescent changes, would be very beneficial. Such sensors would have tremendous impact in structural applications ranging from automotive to aerospace to residential and commercial construction.